A modern trend in the design of antennas for wireless devices is to combine two or more antenna elements into an antenna array. Each antenna element in such an array should have a small footprint, a low level of mutual coupling with neighboring elements, a low element return loss, a low axial ratio (in case of circular polarization), and a large frequency bandwidth. For a typical antenna element in an antenna array, however, these requirements are typically at odds with each other. For example, the larger the bandwidth and the larger the size of an antenna element, the stronger will be the mutual coupling between the antenna element and its neighboring elements in the antenna array.
FIG. 1 depicts a conventional patch antenna element 100 for use in an antenna array. Patch antenna element 100 includes a driver patch 110 and a ground plane 130, separated by a dielectric substrate 120. An input signal having a given wavelength λ is inserted via a microstrip feed line (not shown) connected to the driver patch 110. The length L of the patch is typically selected to be ½ of the wavelength, so that the patch resonates at the signal frequency of the signal and thereby transmits the desired wireless signal. At low frequencies, however, the wavelength λ can be very long, and the patch antenna dimension L can become quite large.
A known technique to reduce the size of the patch antenna element is to select a dielectric substrate 120 with a very high permittivity ∈S (e.g., ∈S=6 to 20 relative to air). The high permittivity substrate reduces the resonant frequency of the patch antenna element 100 and thus allows a smaller driver patch to be used for a given signal frequency f. More specifically, for the patch antenna element shown in FIG. 1, and for a given signal frequency f, the length of the driver patch is conventionally selected to be inversely proportional to the square root of the permittivity ∈S of the substrate 120. For example, if the length L were nominally 1 cm for a substrate permittivity of 1, the length L could be reduced to 0.5 cm for a substrate having a permittivity of 4 were used, or to 0.33 cm for a substrate having a permittivity of 9.
The effect of the increased dielectric permittivity is to raise the capacitance between the patch 110 and ground plane 130 and thereby to lower the resonant frequency. Unfortunately, the reduced antenna volume decreases the bandwidth of the antenna element and causes difficulties with impedance matching. Using conventional design methods known to those of skill in the art, the bandwidth may be improved to some extent by increasing the thickness of the substrate. A thicker substrate, however, introduces additional problems by (i) increasing the antenna's cost; (ii) increasing the antenna's mass (or weight), which may be unacceptable in space applications; and (iii) exciting unwanted electromagnetic waves at the substrate's surface, which lead poor radiation efficiency, larger mutual coupling between antenna elements and distorted radiation patterns. Moreover, a very thin substrate is conventionally used for the feed network—including, e.g., the microstrip feed line (not shown)—and it is preferable to build antenna elements with the same substrate as that used for the feed network.
FIG. 2 depicts another known technique to improve the bandwidth of an antenna element by adding a parasitic patch above the driver patch, resulting in a “stacked patch antenna.” Stacked patch antennas have been described in the article entitled “Stacked Microstrip Antenna with Wide Bandwidth and High Gain” by Egashira et al., published in IEEE Transactions on Antennas and Propagation, Vol. 44, No. 11 (November 1996); and in U.S. Pat. Nos. 6,759,986; 6,756,942; and 6,806,831. As shown in FIG. 2, a conventional stacked patch antenna 200 includes a ground plane 250 supporting a dielectric substrate 240, a driver patch 230, a foam dielectric 220 having a permittivity similar to air, and a parasitic patch 210 (also known as a “driven patch” or “stacked patch”). A signal to be transmitted is input to the driver patch 230. The parasitic patch 210 is electromagnetically coupled to the driver patch 230 and therefore resonates with it. The additional resonance provided by the parasitic patch 210 improves the operational frequency of the stacked patch antenna 200 and increases the bandwidth of the antenna. In conventional stacked patch antennas, however, parasitic patch 210 must be fairly large in comparison with driver patch 230, as reflected in FIG. 2, due to the relatively low permittivity of the foam dielectric 220. As a result, when stacked patch antenna elements are combined in an antenna array, adjacent elements exhibit a strong mutual coupling effect on each other, which negatively impacts antenna element and array gain, radiation patterns, bandwidth and scanning ability of antenna array. Furthermore, in view of recent trends in miniaturization, conventional stacked patch antennas are still too large.
Thus, in conventional designs, the performance of a patch antenna is compromised in order to reduce the size of the antenna. Accordingly, there is a need for a patch antenna that requires a smaller volume than existing antennas without compromising the performance of the antenna. The present invention fulfills this need among others.